The present invention relates generally to energy sources for physically small, low power systems.
Mehregany, "Microelectromechanical Systems," IEEE Circuits and Devices, July 1993, pp. 14-22, for example, describes integrated circuit fabrication processes which can be used to produce micronsized mechanical devices for combination with integrated electronics. As described by Mehregany, microelectromechanical systems (MEMS) techniques have been used to fabricate a great variety of mechanical microstructures, including beams, diaphragms, grooves, orifices, sealed cavities, pyramids, needles, springs, complex mechanical suspensions, gears, linkages, and micromotors. MEMS are fabricated on silicon, for example, by appropriately combining etch masks and etch-stop patterns with anisotropic etchants. Surface micromachining relies on forming specific structural parts of a device over layers or regions of a sacrificial material which is later dissolved in a chemical etchant that does not attack the structural parts. MEMS devices require a power source for operation which is often in the form of an electrical power supply.
The availability and durability of power supplies are becoming increasingly significant factors for many electronic systems. Many microelectronic systems are requiring less power than in the past because of more energy-efficient designs, but they require some power nonetheless. These power requirements can create difficulties for systems that include electronics but are not easily recharged by outside sources. Such systems include electronic storage and data manipulation systems such as sensor systems deployed in remote areas or areas to which access is not readily available, and multi-function (balance calculating, inventory taking, etc.) credit cards and instruments, for example.
Wahlstrom, U.S. Pat. No. 4,054,826, describes an apparatus for charging batteries by converting random mechanical motion into electrical energy. Diodes are positioned in the circuit so that as electrical charge passes between capacitors, the current formed thereby passes through the battery and charges it. In one embodiment, two variable capacitors and a bias supply are connected to a battery with the variable capacitors having their capacitance varied in an alternating manner by the physical displacement of a conductive member that forms one plate in each of the capacitors. From the text and description, it appears that the electrical connection to the conductive member is through a conductive housing in which the conductive member is displaced. Consequently, free movement and friction of the conductive member in the housing may reduce the strength of the electrical contact between the conductive member and the conductive housing. In another embodiment, the capacitance is varied by having a conductive liquid, such as mercury, move alternately between two sets of capacitor plates. Mercury liquid is environmentally toxic. Liquids in general have disadvantages of potentially high vapor pressure, dampening viscosity, and stress on the container walls resulting from a high coefficient of thermal expansion.
Furthermore, the above embodiments apply to electromechanical systems which are assembled by conventional "pick-and-place" techniques using discrete manufactured parts. These techniques are too coarse and costly for use in micropower supplies compatible with MEMS devices.